In the electrowinning of metals having oxidation potential greater than hydrogen overvoltage, presence of impurities in the electrolytic solution causes marked impairment of electrolysis efficiency and sometimes it makes the electrolysis impossible. Therefore, usually there is provided a step for purifying the electrolysis solution wherein impurity metal ions are removed. In the case of electrowinning of zinc, the zinc sulfate electrolysis solution is purified by removing copper, cobalt, cadmium, etc. Of these, copper is precipitated by addition of zinc powder and cobalt is precipitated by addition of zinc powder and arsenic oxide (As.sub.2 O.sub.3) and the precipitates are removed.
It is necessary to determine the concentrations of the impurities to be removed prior to purification. For the measurement of low concentration impurity metals, (a) colorimetric method using a coloring reagent, (b) measurement of redox potentials, (c) atomic absorption analysis, (d) Inductively coupled Plasma Atomic Emission Spectrometry (ICP) emission spectrophotometry, etc. are generally employed. In the case of zinc sulfate solution, however, manual chemical analysis is solely resorted to for the measurement of concentrations of copper and cobalt ions, since the zinc sulfate solution is a relatively viscous, acidic or weakly acidic solution with super-saturated zinc and is liable to clogging of conduits with the deposition of salts and automatic and continuous analysis is impossible. That is, samples are taken for the determination of concentration of copper and cobalt each time and the samples are subjected to classic colorimetric analysis or titration.
In the classic colorimetry, color of a sample is compared with a reference by human eye. It takes a long time and the results are not so accurate. Thus, it is an obstacle for automating the electrowinning process.
There has been an attempt to overcome this defect of the prior art. It is called "flow injection method". That is, there is provided a flow of a reagent mixture. The sample taken from the electrolysis solution is added to this flow when measurement is desired and the concentration of the object impurity is measured spectrophotometrically for instance.
This method has the following defects. 1. The sample is dispersed in a reagent solution and thus the sample is highly diluted and thus the measurement of a very slight amount of the impurity is difficult. 2. Expensive coloring reagent is wasted.
This invention is intended to overcome the above-mentioned various problems and provide improved method of measurement of the impurities in the electrolysis solution for electrowinning of zinc and further to provide an automatic purifying system for electrowinning electrolysis solution for electrowinning of zinc.